** Deb Beutler writes -
>
> I would be surprised to see a large number of birds arriving early
> due to El Nino. Spring migration and other breeding behaviors seem to be
> controled by day length, not temperature, particularly in passerine birds.

Yes, I remember Tony Erskine did a study of plant blooming phenology vs. arrival dates
of various bird species across boreal Canada quite a while ago now, and found little to
no correlation.

<snip>

> Birds use day-length as a signal because it is a good predictor of
> spring, better than temperature. As we all know, the temperature fluctuates
> greatly, day to day and year to year. The birds need a reliable predictor
> of upcoming warm weather and increasing day length does that. El Nino and
> other large-scale weather patterns alter temperature but have no effect on
> day length.
> Obviously, this works best for birds that breed and winter in the
> north temperate zones and restrict breeding to the spring and summer months.
> Birds that winter along the equator may have an additional factor, probably
> a circannual rythmn, that makes them more sensitive to sunlight a certain
> time. Birds that winter south of the equator may be "turned on" by shorter
> day lengths. Birds that nest in unpredictable environments or depend on
> unpredictable food source (such as crossbills) may use a variation of this
> pattern or may use a totally different system we haven't yet found.

Temperature apparently enters into the equation for my backyard N. Saw-whet Owl.
In warm winters such as this one, he starts his courtship tooting much earlier than
in colder years. He started about two weeks ago this year. Last year it wasn't until
late March!

> Variation in arrival dates arise due to conditions along the
> migration route. The day length starts the process. Then the bird starts
> migrating. If food supplies along the route are below average, the bird
> might have to spend more time at stopovers. If food is abundant, the trip
> may go faster. However, I doubt the process sped up by several weeks.

However, as Michael P. notes, if the bird started out from a farther north location
than normal due to a mild winter, then it is quite conceivable that its northbound
trigger would have it displaced that much farther northward when the hormones
start pumping.

> An alternate explanation for apparent "early migrants" are winter
> stragglers that never left and were never discovered. It is possible that
> every winter a few birds just don't leave; winter weather usually takes care
> of them. However, during mild winters, particularly El Nino winters, they
> can survive and are "found" on those early dates.

Also possible.

> I predict that most birds will be right on time.

Probably. But there are bound to be plenty of early outliers from the median
dates of arrival this spring.

Incidentally, the current synoptic pattern over the western US is quite
conducive to birds hitching a wind-assisted ride since the warm El Nino winds
have been out of the south through a fairly deep layer of the atmosphere for
about a week now... all the way from Baja California (oh no, not Baja again...).

- Jack




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Jack Bowling
Prince George, BC
jbowling at direct.ca